JavaScript (Node.js), 43 bytes
x=>~(g=s=>~s.indexOf(x,1)||g(s+i++))(x,i=1)
Using range, fail 11 (expect 13, get 1)
JavaScript (Node.js), 49 bytes
x=>~(g=s=>~s.search(x)||g(s+ ++i))(i='')+x.length
-4 from Arnauld
Not using the 10
l4m2
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